^Like I keep saying, people in real life can potentially survive that kind of impalement injury with prompt medical attention, and the medics were on the scene pretty promptly. I don't think there needs to be any more complicated explanation than "Fury lied."
While it's of course possible to explain it away like that, I think this would somewhat diminish the emotional impact of Coulson's death in the movie. That's why I suspect Whedon et al. have something a little more complicated in mind.